1. Field of the Invention
The invention concerns a method of making a lever-type cam follower out of thin-walled sheet metal, particularly for a valve train of an internal combustion engine, said cam follower comprising two parallel side walls connected by a crossbeam, an underside of the crossbeam having a support for at least one gas exchange valve on one end, said support being limited by guide walls extending parallel to the side walls, and the invention also concerns a lever-type cam follower having the aforesaid features.
2. Background of the Invention
A cam follower of the pre-cited type is known from U.S. Pat. No. 5,016,582 considered to be generic art. In this cam follower, the support for a valve stem is realized by a complex double bending procedure. Due to the higher mass lying relatively far from the fulcrum, an unnecessarily high mass moment of inertia can be expected. Further, drawbacks with regard to design space are evident to a person skilled in the art. These drawbacks arise from the fact that a doubled wall is situated on each side of the support. It is also possible for cracks to be formed in the bent region.
Another lever-type cam follower of the pre-cited type made of sheet metal is disclosed in DE-OS 198 30 427. Although the above-mentioned multiple bend design is not used in this cam follower, the support for the valve stem is realized through bent-over tabs that have first to be punched out from an end of the cam follower. The ends of the tabs are fixed by welding on the underside of the cam follower. It must be observed that the punching, bending and welding of the tabs involves a relatively great amount of fabrication work. Besides this, cracks can occur in the bent region. The use of welding results in an undesired input of heat into the material which can lead to microstructural changes therein. Furthermore, the weld joint constitutes a quality risk. It must also be noted that the bent region of the tabs unnecessarily lengthens the cam follower in axial direction. This can cause problems with the design space during installation, or again, the mass moment of inertia of the cam follower is unnecessarily increased.
The object of the invention is therefore to propose a method of making a lever-type cam follower, and a lever-type cam follower in which the initially stated drawbacks are eliminated by simple measures.
According to the invention, this object is achieved by the features recited in the characterizing part of claim 1, and by the feature of the characterizing part of independent claim 5.
The method proposed by the invention comprises the following work steps, to which further intermediate steps may be added:
a) punching out a blank from cold or hot rolled strip;
b) making the support and the guide walls by cold forging by applying a stamping die to the underside laterally of the guide walls to be formed, said stamping die acting in such a way on the underside and having such a shape that a region of the blank situated under the stamping die is thinned out and the guide walls are formed by a backward plastic flow of the thinned-out material, and
c) forming the side walls with the crossbeam enclosed by bending long sides of the blank at a right angle to the original blank.
Alternatively, it is basically within the scope of the invention to make the guide walls by cold forging.
By the proposed cold forging is meant, in particular, an extrusion molding and stamping procedure. This has the advantage that relatively clean surfaces with a relatively higher end precision can be obtained. Moreover, a desired increase of hardness is obtained in the region of deformation.
The material of the guide walls, that are thus made for the first time in the art in a sheet metal cam follower by cold forging, is obtained by plastic flow out of material situated on the side of the guide walls, as seen in a top view of the underside. The material of the blank situated under the stamping die is thinned out. By the use of adequate counterdies, roughly indicated in the drawing, is not only a geometrically very accurate support defined but also the upper side in the region of the support.
A particular advantage of the method of the invention is that an excellent stiffening of the cam follower in the region of the support is to be noted. The cam follower requires less design space in longitudinal direction than prior art cam followers. Further, a reduction of width in the region of the support is also possible because double bending can be dispensed with. The cam follower has only a relatively low weight and, due to the absence of material accumulation at a distance from the fulcrum, a favorable mass moment of inertia is obtained. In addition, it is clear to a person skilled in the art that the crack formation mentioned in the introductory part of the description no longer occurs. The cost of manufacturing can be distinctly reduced compared to the initially commented cam followers.
The lever-type cam follower is particularly envisaged as a rocker arm, a finger lever or an oscillating arm. An adequate cam-contacting surface in cam followers that are not activated by tappet push rods can be made optionally as a rolling-bearing mounted roller or as a sliding surface.
According to another feature of the invention, the support has a convex shape, as viewed in the longitudinal direction of the cam follower, so that, for example, the excursion area on the cam follower for the contacting end of the valve stem is reduced. On the other hand, this also creates a particularly stiff support. It is likewise conceivable and within the scope of the invention to implement other shapes of valve stem supports depending on the particular use.
It is particularly advantageous if the method steps proposed in claim 1, to which further intermediate steps may be added, are all carried out on one and the same machine. This results in low manufacturing costs and in a concomitant reduction of unit time. Furthermore, the weight of material used is reduced compared to the prior art commented in the introductory part of the description because the connections for the bent tabs (only when made of strip material) are dispensed with.
According to a further proposition of the invention, the guide walls are configured so as to have a height of at least 2.5 mm relative to the support situated therebetween. However, it is also conceivable to provide different heights, for example, also smaller heights, depending on the particular use. Thus, an excellent guidance of the cam follower can be obtained on every type of valve shaft end. After it has been mounted, the cam follower can no longer slip off the end of the valve stem before the initial ignition of the engine, and a smooth gas exchange is achieved when the engine is in operation.
Alternatively to the method claim 1, independent claim 5 proposes a cam follower whose guide walls are formed by cold forging.